定向铸造镍基高温合金Hf夹杂的特征及成因

Characteristics and formation mechanism ofHf inclusions in directionally casted Ni-based superalloy

采用扫描电镜、透射电镜和能谱技术,详细分析DZ125和DZ22镍基高温合金中富Hf夹杂物的形貌、结构及成分,开展不同Hf添加量的镍基合金凝固组织特征的模拟实验探究,阐明Hf夹杂物的形成机理。结果表明:DZ125和DZ22合金铸件的边缘均存在不规则形态的夹杂物,其中白亮夹杂物为HfO_(2),与之伴生的灰色夹杂物中含有Al_(2)O_(3)。镍基高温合金中的Hf元素聚集倾向强,Hf含量较高时可形成百微米以上的大尺寸富Hf相,随着Hf含量降低,富Hf相减小且分布更为均匀。高温下Hf与熔体中的氧、陶瓷型壳/型芯氧化物具有高的自发反应驱动力,当Hf分散不充分时,Hf倾向于向氧化物陶瓷相/熔体的界面聚集,在高温下与陶瓷中的Al_(2)O_(3)和硅溶胶黏结剂中的SiO_(2)发生反应,从而导致铸件表层HfO_(2)夹杂物的形成。

Scanning electron microscopy,transmission electron microscopy and energy disperse spectrum were used to analyze the morphologies,microstructures and compositions of Hf-rich inclusions in DZ125 and DZ22 Ni-based superalloys.The solidification microstructures of Ni-based alloys with different Hf additions were investigated by simulation experiments,and the formation mechanism of Hf inclusions was clarified.The results indicate that there are irregular inclusions on the edges of DZ125 and DZ22 alloys.The white inclusions are HfO_(2) phase,and the accompanying gray inclusions mainly comprise Al_(2)O_3.The Hf has a strong tendency to accumulate in the alloy,and large size Hf-rich phase of above 100 μm can form in the alloy when the Hf content is high.With the decrease of Hf content,the Hf-rich phases become smaller and distributed more evenly.At high temperature,Hf has a high strong driving force of spontaneous reaction with oxygen in the alloy melt and ceramic shell/core oxides,leading to the formation of HfO_(2) inclusions on the casting surface.When the dispersion of Hf is insufficient,Hf tends to segregate at the interface of oxide ceramic phases and melt,reacting with Al_(2)O_(3) in the ceramic and SiO_(2) in the silica sol binder at high temperature,resulting in the formation of HfO_(2) inclusions on the casting surface.